Fix MIPS sigaction build.

Building MIPS sigaction (for ABIs other than o32) fails because of
"'restore_rt' used but never defined", arising from static functions
being defined in asms and referred to from C code.  There is no
corresponding -W option for that warning, so this patch uses
-Wno-error for building sigaction.c.

	* sysdeps/unix/sysv/linux/mips/mips64/Makefile
	[$(subdir) == signal] (CFLAGS-sigaction.c): New variable.
	[$(subdir) == nptl] (CFLAGS-sigaction.c): Likewise.